Shadow file return LOCK on 2nd field

Hi,

Recently i saw the LOCK on /etc/shadow files.
1. May i know how can configure this?
2. Is there any reference i can check on this? Like what it does?

I have checked on shadow man pages there is no written for this info.
If i lock the user, it will show the 2nd field value with
Username:!hash:::::

Above scenario i seen is as below
Username:LOCK:::::